This disclosure is directed to a method and apparatus for determination of the compressive strength of formations which are penetrated by a well borehole. More particularly, it is a method and apparatus for use during open hole drilling procedures, particularly those which involve positioning a formation test tool in the well. This takes advantage of the manner in which a formation test tool is constructed to provide the additional benefit of the test described herein. By way of background, the formation test tool is normally constructed with a snorkel which extends from one side of the formation test tool sonde so that the snorkel is able to penetrate into formations aligned adjacent to the formation tester. Moreover, the formation tester is normally held in location by means of back-up shoes. Thus, the snorkel extends on one side of the tool in response to urging of a hydraulic system within the tool. Above and below the snorkel, there are separate back-up pistons. For instance, one might be two feet above the snorkel and the other back-up shoe is about two feet below the snorkel. The two back-up shoes are included so that they can extend in the opposite direction (diametrically opposite the snorkel) and prop the formation test tool when the snorkel is extended. The back-up shoes and opposing snorkel particularly reduces differential sticking of the sonde against the sidewall of the borehole during operation. When the snorkel is extended, the two back-up shoes shoes are also extended. When the snorkel is retracted, both of the back-up shoes are normally retracted. This assures a desirable sequence of operation where the snorkel is first extended and then retracted while the snorkel obtains the intended data from its operation.
Measurement of the compressive strength of the rock is an important factor. This data along with other information assists in determining whether or not the formation is permeable. This is particularly helpful since the back-up shoes are located on the opposite side of the snorkel, spaced immediately above and below, and are likely to be contacted against the same formation. Thus, if the snorkel is extended into a particular formation, it is helpful to know the compressive strength of that formation. The compressive strength test is performed during routine operation where the snorkel is extended which motion is accompanied by extension of the two back-up shoes on the opposite side of the formation test tool. To this end, the back-up shoes are equipped with a protruding punch and an adjacent back-up plate, both to be described. The punch and back-up plate both engage the wall of the borehole, and hydraulic pressure readings are taken. The instant of rock failure in response to the urging of the punch will be noted by a change in hydraulic pressure. Separate tests can be performed with each of the two back-up shoes. Thus, two separate data are obtained whereby the failure of the rock making up the formation is noted and the compressive strength of the rock in the formation is then determined. A typical operation sequence involves extension of the snorkel after the extension of the two back-up shoes on the opposite side. They are extended until they touch the adjacent formation when they are stopped and held at that position. One of the two is then further extended so that the protruding punch penetrates and breaks the rock whereby a pressure drop is noted in the hydraulic system extending the back-up shoe. The puncture wound formed in the formation causes this drop in pressure. That back-up shoe is then forced further toward the formation so that the back-up plate then contacts the formation and provides a sure footing. The routine is repeated for the other of the two back-up shoes so that both data are obtained.